This invention relates to telephones used in connection with, e.g., cellular telephone systems. In particular, it pertains to the selective enablement and disablement of specific sets of frequencies while a telephone is physically "roaming" among service areas, with an override for emergency numbers such as 911.
Two concepts are of particular pertinence to the invention: the home frequency set of a given telephone unit, and the home system identification code, or SID, of the unit.